PROJECT SUMMARY Thirdhand smoke (THS) is the persistent toxic residue of tobacco smoke pollutants that attaches to surfaces, remains in dust, and becomes embedded in environments where secondhand smoke (SHS) has been released into the air. Even when parents are diligent in attempting to shield their children from tobacco smoke, exposure to SHS (SHSe) and THS (THSe) still occurs. Residue from previously smoked tobacco at home or in a car creates a pervasive reservoir of tobacco smoke pollutants that are within reach of a child's hand such as on carpets, furniture, clothes, and toys, and breathed in dust. Homes of smokers become reservoirs of these pollutants that persist for years, become more toxic with age, and are later reemitted into the gas phase, or react with common ambient oxidants and other environmental compounds to yield secondary pollutants such as carcinogenic tobacco-specific nitrosamines. The clinical effects of THSe are unknown because prior research has examined children exposed to both SHS and THS. Studies exclusively focusing on THSe are needed. Thus, we have conducted in-vitro, animal, and pilot pediatric studies that demonstrate that THSe is associated with high levels of inflammatory cytokines, respiratory illnesses, and oxidative stress including DNA damage, in mice. We have found even in the presence of smoking bans, children still have high levels of THS pollution on their hands and that nonsmokers' homes have high THS pollution levels. However, in the absence of large-scale, rigorous human studies, conclusions about the clinical effects of THSe cannot be made. Thus, we propose an innovative prospective longitudinal cohort study that will move the field forward and away from the laboratory to a pediatric clinical setting as the first pediatric study to use biochemical validation to identify children exposed to THS-only over time. We will screen 1000 children from the pediatric emergency department, a setting which cares for children with varying tobacco smoke exposure (TSE) levels and illnesses. Using strict screening and chemical analysis techniques, we will identify and compare children that comprise three distinct TSE groups: 1) THSe only group: live with nonsmokers, no reported SHSe, confirmed negative/low levels of SHSe, elevated levels of THSe; 2) Mixed SHSe+THSe group: live with smokers, reported SHSe, confirmed moderate/high levels of SHSe, high levels of THSe; and 3) No TSE group: live with nonsmokers, no reported SHSe, confirmed negative levels of SHSe and THSe. The prevalence and levels of THSe, demographics, and exposure and illness patterns of these groups will be compared over time. Next, we will disentangle which environmental sources contribute to THSe levels and the association between THSe and inflammatory (i.e., cytokines) and oxidative (i.e., DNA, lipid, protein oxidation) biomarkers and TSE- related clinical illnesses. Results will provide unique insights into how THSe and sources of tobacco smoke pollution affect the inflammatory, oxidative, and clinical responses in children. The potential impact of these findings is substantial as currently, the level of clinical risk attributable to THSe is unknown.